| Title | Changes in Production Capacity at Hellisheiði and Nesjavellir Geothermal Power Plants |
|---|---|
| Authors | Pálmar SIGURÃSSON, Gunnar GUNNARSSON, Ãris Eva EINARSDÓTTIR, Iwona Monika GALECZKA, Þorsteinn Ari ÞORGEIRSSON |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | geothermal, production, capacity, enthalpy, mass flow rate, productivity curve, Hellisheiði, Nesjavellir, Hengill, Iceland |
| Abstract | There are two geothermal power plants built around Hengill central volcano, at Nesjavellir and Hellisheiði. Thermal production began at Nesjavellir in 1990 and electrical production in 1998. Nesjavellir power plant reached its current capacity in 2005, with installed capacity of 120 MWe and 290 MWth. Production at Hellisheiði began in 2006 and since 2011 the installed capacity has been 303 MWe and 133 MWth. Production is monitored at both power plants. The monitoring has increased since the beginning of production and is substantial. Changes in production capacity are monitored with two different and independent methods. One method relies on measurements of power outputs of geothermal wells and comparison of those measurements. The other method simulates and forecasts the changes in production over a certain period, using production data, pressure monitoring, temperature monitoring and more. The outputs of the two independent methods are then represented as two figures and as a combined result, the latter with uncertainties of both methods. The power output and chemical composition of production wells are measured up to twice a year. The method used is a tracer fluid test (TFT). There are two different types of tracers used, one for the vapor phase and one for the liquid phase. Both tracers are injected in the stream of geothermal fluid coming up a production well, close to or inside the wellhead. Samples of the geothermal fluid are taken further downstream and the dilation of both tracers measured. From there the mass flow of each phase, and therefore the enthalpy, is known. Productivity curves for each well are fitted to the measured data. The annual changes in production capacity are then calculated by using a productivity curve, enthalpy and levelized wellhead pressure for each production well. The forecast method relies on newest data on production, power output of production wells, temperature and pressure changes in the reservoir and more. The model itself and input values are update every year. From the input values and a calibrated model, a production capacity is calculated and state of the reservoir estimated. Assumptions are made that there are no make-up wells or any other changes in operation during the predicted period. Therefore, results of the simulations are predicting the state of the geothermal reservoir and production capacity after a certain period. Results from both methods are compared and represented as a single value for changes in production capacity. The value is represented with limits of uncertainty, estimated from both methods. Results from the methods are updated once or twice a year and utilized in decision making processes in terms of the need for drilling make-up wells, actions related to more evenly distributed production and more. |